1. Field of the Invention
The present invention relates to audio subband encoded signal decoders which conceal errors produced when a signal of low reliability is reproduced from a recording medium such as a tape or a disk where an audio subband encoded signal is recorded, or when a subband encoded signal is reproduced through a transmission system of low reliability.
2. Prior Art
Recently, audio subband signal encoders and decoders for decoding such encoded signals have attracted attention as devices which provide low bit rate coding of a high quality audio signal.
Conventional audio subband signal encoders and decoders are disclosed, for example, in an article entitled "LOW BIT-RATE CODING OF HIGH QUALITY AUDIO SIGNALS. AN INTRODUCTION TO MASCAM SYSTEM" by G. Theile et al., EBU REVIEW-TECHNICAL, No. 230, pp. 71-94, August 1988.
Conventionally, an audio subband signal encoder divides a received digital audio signal into a plurality of subband signals (16-32 subband signals corresponding to the critical band of a human ear characteristic), sections the respective subband signals into frames of a constant time interval of 5-20 ms, determines range data in accordance with the maximum signal level in a frame, reduces and compresses a quantity of data (the number of bits) by removing unnecessary components of a subband signal which a human being cannot hear on the basis of psychoacoustic rules in accordance with the input signal. To this end, it calculates bit allocation data indicative of the number of allocation bits of each subband signal, calculates sample data obtained by normalizing the respective subband signals with the maximum signal level determined by the range data, and quantizing them with the number of bits determined by the bit allocation data, calculates error detection and/or correction data for protection of the bit allocation data, range data and sample data against errors, and outputs synchronous data for synchronization of each frame, bit allocation data, range data, sample data and error detection correction data as a subband encoded signal.
Conventionally, an audio subband encoded signal decoder separates an input subband encoded signal to frames on the basis of synchronizing data, detects or corrects a possible error in the subband encoded signal using error detection and/or correction data, separates a signal in each frame into range and sample data in each subband on the basis of bit allocation data, resets the range or sample data where an uncorrectable error, if any, occurs, performs dequantization and denormalization using the separated range and sample data in each subband to obtain a subband signal, synthesizes those subband signals to reproduce and outputs a digital audio signal.
However, when an error occurs in the range data of a subband encoded signal, the decoder resets the range data in the subband where the error has occurred, so that the form of the frequency spectra of the audio signal reproduced by synthesizing the subband signals is disturbed to thereby greatly deteriorate the quality of the audio signal.
When an error occurs in the synchronizing data, and the subband encoded signal cannot be separated into frames, the range and sample data in the frame where the error has occurred is all reset, so that the level of the reproduced audio signal is greatly attenuated in the frame where the error has occurred to thereby greatly deteriorate the quality of the audio signal.
When an error occurs in the bit allocation data, and the subband encoded signal cannot be separated into respective subbands, the sample data in the frame where the error has occurred is all reset, so that the level of the reproduced audio signal is greatly attenuated in the frame where the error has occurred to thereby greatly deteriorate the quality of the audio signal.